Paulo Lopes et al. (Faculté d’Oenologie de Bordeaux)
publish their pioneering work on different closures in the Journal
of Agricultural and Food Chemistry. A summary:

Main Routes of Oxygen Ingress through Different Closures
into Wine Bottles

Paulo Lopes et al. Faculté d’Oenologie de Bordeaux

The main routes of oxygen ingress into wine bottles
through “technical” cork stoppers (Neutrocork),
natural cork stoppers, and synthetic closures (Nomacorc) were
investigated. A comparison was made among closures left uncovered
(controls), closures with the closure-glass interface covered,
and closures completely covered with a polyurethane impermeable
varnish. The oxygen ingress into the bottles was measured by
a nondestructive colorimetric method.

Introduction

During bottle aging, oxygen ingress into wine bottles depends
on the sealing effectiveness of the closures, which differ in
their oxygen barrier properties. Recently, we have shown that
only a bottle sealed with a glass closure by flame (bottle ampoule)
is completely airtight, while more commercial closures are permeable
to oxygen.

Generally, oxygen ingress through closures into wine bottles
is much more important during bottling and in the first month
than in the following months of storage. For natural corks,
this is followed by a gradual decline in oxygen ingress rates
for the remainder of the first year (2-6 µL/day) and a
very low rate of oxygenation in the 24 months thereafter (0.1-2
µL/day). Screw caps and “technical” cork stoppers
display a consistently low level of oxygen permeation during
storage (less than 1µL/day). In contrast, synthetic closures,
Nomacorc and Supremecorc, continue to
exhibit high oxygen permeation rates, 6 and 13 µL/day,
respectively.

Given their high oxygen permeation rates, the use of synthetic
closures resulted in wines with a tendency to lose fruit attributes
and develop oxidized, “wet wool”, and toasty aromas
prematurely. In contrast, too little oxygen has been linked
with the presence of undesirable struck flint/rubber (reduced)
aroma characters, more noticeable in screw-cap-sealed wines.
Generally, wine sealed with natural corks displayed intermediate
performances.

This study complements and extends the results of previous
research on oxygen barrier properties of closures using a nondestructive
(i.e., without opening the bottles) colorimetric method. This
method screens oxygen ingress through closures into indigo carmine
bottled solutions that gradually changes color from yellow to
indigo as oxygen reacts with the reduced indigo carmine. Our
purpose was to investigate the different routes of oxygen ingress
through different cork stoppers and synthetic closures during
bottle aging.

Materials and methods

For each closure type three different “treatments”
were set up. Indigo carmine bottled solutions were sealed normally
(i.e., uncovered). For another set of closures the closure-glass
interface was covered with an impermeable polyurethane varnish.
A thin layer of varnish was applied to the internal surface
of the bottleneck before closure insertion; the external closure-glass
interface was then also covered with varnish. In this way, oxygen
ingress between the closure and the bottleneck was prevented
and only the permeation throughout the closure’s body
was measured. Finally, another set of closures were completely
covered with polyurethane varnish and glass (20 x 20 mm). Therefore,
only oxygen within the closures able to ingress into the bottles
was measured.

All bottles were left upright for 24 h and then stored horizontally
for 24 months under a constant temperature of 20 ± 1
ºC and a relative humidity of 65 ± 1%.

After 24 months of horizontal storage, oxygen ingress amounts
through uncovered, interface-covered, and fully covered Neutrocork
technical corks were 0.7, 0.7, and 0.6 mL of oxygen, respectively,
without significant differences between the three treatments.
The results obtained in this study seem to show that oxygen
diffuses from the Neutrocork internal structure due to the compression
in the bottleneck, mainly during the first month.

Uncovered, interface-covered, and fully covered natural corks
displayed oxygen ingress of 1.4, 1.3, and 1.3 mL over 24 months
of storage, respectively. There were no significant differences
between the three treatments.
When natural cork stoppers are compressed in the bottleneck
immediately after bottling, the air pressure in the cells ranges
from 0.6 to 0.9 MPa. Therefore, air at atmospheric pressure
(0.101 MPa) is unable to enter into the bottles through the
cork or between the cork-bottleneck interface.
Theoretically, natural corks (44 mm length and 24 mm diameter)
contain 3.4-3.6 mL of oxygen within their structure. We have
shown that 1.3 mL of oxygen diffuses from the natural corks
into the bottles, which represents 36-38% of the theoretical
total oxygen within their cell structure.

With regard to the Nomacorc synthetic closures, the results
showed that uncovered and interface-covered closures exhibited
high oxygen permeation, reaching 2.5 mL of oxygen (limit of
quantification for the method) within approximately 8 months.
In contrast, closures fully covered with impermeable polyurethane
varnish allowed ingress of 1.4 mL of oxygen during 24 months.
Uncovered and interface-covered Nomacorc closures were clearly
much more permeable than those fully covered. These data clearly
indicate that atmospheric oxygen can ingress throughout Nomacorc
synthetic closures, mainly after the first month in the bottle.